1. Field of the Invention
The present invention relates to a liquid crystal display device and a driving control method thereof, and particularly to a liquid crystal display device of an active matrix type which uses a plurality of thin-film transistors as switching elements, and a driving control method thereof.
2. Description of the Related Art
In recent years, liquid crystal display devices (LCD) for displaying images, text information, and the like are mounted on image pickup apparatuses represented by digital video cameras, digital still cameras, and the like, portable phones, and personal digital assistants (PDA). Further, in place of conventional cathode ray tubes (CRTs), liquid crystal display devices have come to be often used as monitors and displays of information terminals of computers and video apparatuses.
A conventional liquid crystal display device will be explained with reference to the drawings. As an example of the liquid crystal display device, explanation will now be made of the structure of a main part of a liquid crystal display device of an active matrix type.
FIG. 8A is a diagram showing an example of an equivalent circuit of a liquid crystal display panel of a conventional active matrix type. FIG. 8B shows the details of a display pixel part in the liquid crystal display panel of the conventional active matrix type. In this case, explanation will be made of the case of using thin film transistors as switching elements.
As shown in the figures, the active matrix type liquid crystal display panel 100 comprises a plurality of signal lines DL extended in the row direction, a plurality of scanning lines GL extended in the column direction, thin film transistors (hereinafter described as pixel transistors TFT) provided respectively near the cross-points between the signal lines DL and the scanning lines GL, pixel electrodes connected to source electrodes S of the pixel transistors TFT and arrayed in a matrix, common electrodes COM opposed to the pixel electrodes and connected in common, liquid crystal capacitances CLC filled between the pixel electrodes and the common electrodes COM, auxiliary capacitor electrodes ES connected in common and forming part of auxiliary capacitances CS opposed to the pixel electrodes to maintain display signal voltages applied to the pixel electrodes. The pixel transistor TFT has a drain electrode D connected to the signal line DL and a gate electrode G connected to the scanning line GL. The liquid crystal capacitances CLC and the auxiliary capacitances CS serve as display pixels and are driven and controlled by the pixel transistors TFT.
FIG. 9 is a timing chart showing write operation of display signal voltages of the conventional active matrix type liquid crystal display panel to the display pixels. FIG. 9 shows a case of writing a display signal voltage into a display pixel by a field inversion drive system. Normally, it is driven at 30 frames per second, and one frame period is about 33.3 ms. In the field inversion drive system, one screen is over-written for every field of ½ frame period (about 16.7 ms), and the polarity of the display signal voltage is inverted for every one field. FIG. 9 shows a case where the voltage Vcom applied to the common electrode COM and the auxiliary capacitor electrode ES is constant. Needless to say, this voltage Vcom may be controlled to be inverted in correspondence with inversion of the display signal voltage.
As shown in FIG. 9, a display signal voltage which is set to invert its polarity with respect to a predetermined center voltage Vsigc for every field, in correspondence with a video signal is supplied to each signal line DL and is thus applied to the drain electrodes D of the pixel transistors TFT. In FIG. 9, the display signal voltage Vsig of a positive polarity is applied in the n-th field, and a display signal voltage Vsig of a negative polarity is applied in the (n+1)-th field.
Meanwhile, at predetermined timing during the period of applying the display signal voltage Vsig, a scanning signal Vg is supplied to each scanning line GL of the liquid crystal display panel 100 only for a predetermined write time Tw and is applied to the gate electrodes G of the pixel transistors TFT. In this manner, the pixel transistors TFT are turned into ON-status, so that the drain electrodes D and the source electrodes S are conducted to each other, respectively, thereby to apply a display signal voltage Vsig to the pixel electrodes. The potential difference between the display signal voltage Vsig applied to the pixel electrodes and the voltage Vcom applied to the common electrodes is a liquid crystal application voltage Vp. This voltage is applied to the liquid crystal molecules filled between the pixel electrodes and the opposite electrodes, their orientation status is changed to light permeability, thereby to change the image. Applied charges are maintained until the write timing in the next field, by the liquid crystal capacitances CLC and the auxiliary capacitances CS. However, as shown in FIG. 9, the applied charges decrease due to leakage currents form the pixel transistors and the auxiliary capacitances CS, so that the absolute voltage Vp of the liquid crystal application voltage Vp decreases.
In case where the thin film transistor is used as a switching element as described above, it is known that there appears a phenomenon that the liquid crystal application voltage Vp decreases only by ΔV at the timing when the scanning signal VG drops, i.e., at the timing when the pixel transistor TFT switches from ON-status to OFF-status, as shown in FIG. 9. This is caused by the influence from a parasitic capacitance CGS between the gate electrode G and the source electrode S of the pixel transistor TFT, because the voltage change ΔVg when the scanning signal VG drops changes the potential of the pixel electrode through the parasitic capacitance CGS. This is called a field-through phenomenon, and the ΔV is called a field-through voltage. The field-through voltage ΔV is generally expressed by the next expression.ΔV=CGS×ΔVg/(CGS+CLC+CS)  (1)
As shown in FIG. 9, the field-through voltage ΔV is constantly generated in the negative-polarity direction, so that a direct current voltage component is generated in the liquid crystal application voltage Vp due to the difference in the positive-negative voltage from the common electrode voltage Vcom. This component is applied to the liquid crystal. In this manner, drawbacks are caused, i.e., flickering and seizing phenomena occur inviting deterioration of display quality, and deterioration of liquid crystal is accelerated resulting in lower reliability concerning the liquid crystal display device. The direct current voltage component is substantially the value of about the field-through voltage ΔV.
Conventionally, to restrict these drawbacks, the method as follows is adopted. That is, as shown in FIG. 9, the common electrode voltage Vcom is corrected by a voltage (offset voltage: about ?ΔV) which cancels the direct current voltage component, so that the positive and negative voltages are equalized substantially with respect to the common electrode voltage Vcom of the liquid crystal application voltage Vp, thereby to restrict the influence from the field-through voltage ΔV.
The liquid crystal capacitance CLC is not a constant value and has a characteristic that it changes on the basis of the voltage applied to the liquid crystal. This is based on dielectric anisotropy of liquid crystal. FIG. 10 is a graph showing an example of change characteristic of the dielectric constant (relative dielectric constant) of liquid crystal with respect to the applied voltage. From this graph, it can be understood that the dielectric constant of liquid crystal increases so that the liquid crystal capacitance CLC increases, when the applied voltage is high, while the dielectric constant decreases so that the liquid crystal capacitance CLC decreases, when the applied voltage is low or in a state where no voltage is applied. Hence, based on the expression (1), the field-through voltage ΔV changes in accordance with the display signal voltage Vsig applied to the pixel electrode. In a state where the applied voltage is low, the field-through voltage ΔV increases, while the field-through voltage ΔV decreases in a state where the applied voltage is high.
Conventionally, the response of liquid crystal to the applied voltage is slow, and therefore, the capacitance value of liquid crystal at the time when the scanning signal VG drops substantially corresponds to the display signal voltage applied during a just preceding field period.
Therefore, change of the liquid crystal application voltage Vp due to the field-through voltage ΔV cannot be excellently cancelled over the entire change range of the display signal voltage Vsig, to restrict sufficiently the influence thereof, only by the method of correcting the common electrode voltage Vcom by a constant offset voltage, as shown in FIG. 9.
Hence, conventionally, the value of the field-through voltage ΔV is decreased by setting the value of the auxiliary capacitance CS to be large to some extent, thereby to change of the field-through voltage ΔV due to change of the liquid crystal capacitance CLC within the change range of the display signal voltage Vsig. In this manner, deterioration of display quality is restricted. However, the auxiliary capacitance electrode ES forming part of the auxiliary capacitance CS is formed by using a process of forming gate electrode of the pixel transistor TFT, and is formed of an opaque metal layer such as aluminum or the like which is adopted to the gate electrode and the like. Therefore, the forming area of the auxiliary capacitance CS is an area which shuts off transmission of light. Therefore, if the auxiliary capacitance CS is set to be large, i.e., if the area of the auxiliary capacitance electrodes ES is set to be large, there is a problem that the area which shuts off light increases, so that the aperture ratio of the display pixels of the liquid crystal display panel decreases, thereby deteriorating the display quality and increasing the power consumption of the back-light source to attain predetermined luminance.